Hello,
Sorry for revive this topic but I think I had a related issue few days ago.
My setup:
Digital Ocean Spaces with s3:upload_assets and s3:expire_missing_assets.
CDN - Bunny
It seems every rebuild or admin update when it upload assets generate a new CORS rule. It is actually duplicate the original rule. Unfortunately I didn’t copy the console error message but it was something before uploading assets - reached the maximum number 100 of CORS rules so I went to Digital Ocean and check the rules and yeah that was 100. 
So I did the same like @pfaffman, I removed one and it removed all rules. After this the assets uploaded successfully on rebuild.
When I setted up this I followed this instruction here: Configure an S3 compatible object storage provider for uploads which is not contains this line yet.
Adding this line will stop generate duplicated CORS rules? Thank you 